A reliable estimate of travel demand has always been one of the main concerns of transportation agencies. Annual Average Daily Traffic (AADT) is one of the important factors being used in planning, design, and management of roads and facilities. The Federal Highway Administration (FHWA) factoring method is widely used by highway agencies to estimate AADTs for a wide range of roads, which are not possible to be covered by permanent counters. In this method, roads in the same functional class are assumed to have similar traffic patterns, and factors derived from the class can be used to account for seasonal variations of roads within the same class. It has been shown in the literature that since road functional class does not represent the seasonal traffic variation on a road, this method may sometimes produce large errors.
Traffic statistics such as annual average daily traffic (AADT), design hourly volume (DHV), and average daily vehicle distance traveled (ADVDT) are important parameters used by many transportation agencies in their projects. These agencies commit significant financial and human resources to collect traffic data and estimate these parameters.
Traffic Monitoring Guide, (Federal Highway Administration (FHWA), 2001) describes two basic components recommended for traffic monitoring programs: continuous count programs and short-term count programs. The continuous count programs measure the traffic volume using permanent traffic counters. These data are collected at 15-min intervals and stored at 1 hour intervals, for 365 days of the year. Information provided by these counters are used to study the temporal variations of traffic volume such as time-of-day, day-of-week, and seasonal traffic patterns on the roadways, which are used to convert short-term traffic counts (STTCs) into AADTs.
Short-term count programs serve as a comprehensive coverage program and are used for covering the majority of road segments without permanent traffic counters within a jurisdiction. One short-term count is usually collected for a road segment every few years and the collection periods usually vary from 1 to 7 days.
Transportation agencies tend to simply use road functional class as the criteria to assign short-term traffic counts to automatic traffic recorders (ATR) factor groups (FHWA 2001) (Traffic Monitoring Guide, 2001); or they use methods such as regression analysis which require excessive data collection efforts. Further, only the count from most recent year is used in the process of estimating AADT. All historical counts collected beyond the most recent year are ignored.